Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball showing a great flight distance on driver shots. The present invention provides a golf ball comprising a spherical core and at least one cover layer covering the spherical core, wherein when JIS-C hardness of the spherical core is measured at points located at distances of 0% (core center), 37.5%, 75.0% and 100% (core surface) from the core center of the spherical core, a hardness difference (Hs−Ho) between a surface hardness (Hs) thereof and a center hardness (Ho) thereof is 20 or more, a ratio ((H 37.5 −Ho)/(H 75.0 −Ho)) of a hardness difference (H 37.5 −Ho) to a hardness difference (H 75.0 −Ho) is 0.6 or less, and a compression deformation amount (D (mm)) of the spherical core and the 37.5% point hardness (H 37.5 ) satisfy a relation of H 37.5 ≦−9×D+95.

FIELD OF THE INVENTION

The present invention relates to a golf ball showing an excellent flightperformance, in particular, an improvement of a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for improving a flight distance on driver shots, forexample, there are methods of enhancing resilience of a core andcontrolling a hardness distribution of a core. The former method has aneffect of enhancing an initial speed, and the latter method has aneffect of a lower spin rate. A golf ball having a low spin rate travelsa great distance.

For example, Japanese Patent Publications Nos. 3674679 B, 3672016 B,2012-139415 A, and 2012-192158 A disclose a technique of controlling ahardness distribution of the core. Japanese Patent Publications Nos.3674679B and 3672016 B disclose a multi-piece solid golf ball having asolid core, wherein the solid core is formed from a rubber compositioncontaining a base rubber, a crosslinking agent and an organic peroxide,and a mixture of 2,5-dimethyl-2,5-di-t-butylperoxyhexyne and1,1-bis-(t-butylperoxy)-3,3,5-trimethylcyclohexane is used as theorganic peroxide, the mixture is in an amount of 0.1 part to 5 parts bymass with respect to 100 parts by mass of the base rubber, and the corehas a maximum hardness at a portion 3-10 mm inside from the coresurface, and a difference between the maximum hardness and a core centerhardness is 3 or more in JIS-C hardness.

Japanese Patent Publication No. 2012-139415 A discloses a golf ballcomprising a spherical core and at least one cover layer covering thespherical core, wherein the spherical core is formed from a rubbercomposition containing (a) a base rubber, (b) an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofas a co-crosslinking agent, (c) a crosslinking initiator, (d) a salt ofa carboxylic acid and (e) an organic sulfur compound, provided that therubber composition further contains (f) a metal compound in case ofcontaining only (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent, and a content of (d) the saltof the carboxylic acid is 10 parts or more and less than 40 parts bymass with respect to 100 parts by mass of (a) the base rubber.

Japanese Patent Publication No. 2012-192158 A discloses a golf ballcomprising a spherical core and at least one cover layer covering thespherical core, wherein the spherical core is formed from a rubbercomposition containing (a) a base rubber, (b) an α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereofas a co-crosslinking agent, (c) a crosslinking initiator, (d) acarboxylic acid and (e) an organic sulfur compound, provided that therubber composition further contains (f) a metal compound in case ofcontaining only (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms as the co-crosslinking agent.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball showing anexcellent flight performance.

The present invention provides a golf ball comprising a spherical coreand at least one cover layer covering the spherical core, wherein whenJIS-C hardness of the spherical core is measured at points located atdistances of 0% (core center), 37.5%, 75.0% and 100% (core surface) fromthe core center of the spherical core, a hardness difference (Hs−Ho)between a surface hardness (Hs) thereof and a center hardness (Ho)thereof is 20 or more, a ratio ((H_(37.5)−Ho)/(H_(75.0)−Ho)) of ahardness difference (H_(37.5)−Ho) between a 37.5% point hardness(H_(37.5)) and a center hardness (Ho) to a hardness difference(H_(75.0)−Ho) between a 75.0% point hardness (H_(75.0)) and a centerhardness (Ho) is 0.6 or less, and a compression deformation amount (D(mm)) of the spherical core when applying a load from an initial load of98N to a final load of 1275 N to the spherical core and the 37.5% pointhardness (H_(37.5)) satisfy a relation of H_(37.5)≦−9×D+95. Thespherical core having such a hardness distribution provides a loweredspin rate on driver shots, as a result, the golf ball of the presentinvention travels a great flight distance on driver shots.

The spherical core is preferably formed from a rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator and (d) a carboxylicacid and/or a salt thereof. The action of (d) the carboxylic acid and/orthe salt thereof in the rubber composition used for the golf ball of thepresent invention, is considered as follows. The metal salt of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms blended inthe rubber composition is considered to form an ion cluster in the core,thereby crosslinking the rubber molecular chain with metals. By blending(d) the carboxylic acid and/or the salt thereof into this rubbercomposition, (d) the carboxylic acid and/or the salt thereof exchange acation with the ion cluster formed from the metal salt of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, therebybreaking the metal crosslinking formed by the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. This cationexchange reaction easily occurs at the core central part where thetemperature is high, but less occurs toward the core surface. Whenmolding a core, the internal temperature of the core is high at the corecentral part and decreases toward the core surface, since reaction heatfrom a crosslinking reaction of the rubber composition accumulates atthe core central part. In other words, the breaking of the metalcrosslinking by (d) the carboxylic acid and/or the salt thereof easilyoccurs at the core central part, but less occurs toward the surface. Asa result, it is conceivable that since a crosslinking density in thecore increases from the center of the core toward the surface thereof,the core hardness increases from the center of the core toward thesurface thereof.

The present invention provides a golf ball showing an excellent flightperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing the golf ballaccording to the preferable embodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the sphericalcore;

FIG. 3 is a graph showing the hardness distribution of the sphericalcore;

FIG. 4 is a graph showing the hardness distribution of the sphericalcore;

FIG. 5 is a graph showing the hardness distribution of the sphericalcore;

FIG. 6 is a graph showing the hardness distribution of the sphericalcore;

FIG. 7 is a graph showing the hardness distribution of the sphericalcore;

FIG. 8 is a graph showing the hardness distribution of the sphericalcore;

FIG. 9 is a graph showing the hardness distribution of the sphericalcore;

FIG. 10 is a graph showing the hardness distribution of the sphericalcore;

FIG. 11 is a graph showing the hardness distribution of the sphericalcore;

FIG. 12 is a graph showing the hardness distribution of the sphericalcore;

FIG. 13 is a graph showing the hardness distribution of the sphericalcore;

FIG. 14 is a graph showing the hardness distribution of the sphericalcore;

FIG. 15 is a graph showing the hardness distribution of the sphericalcore;

FIG. 16 is a graph showing the hardness distribution of the sphericalcore;

FIG. 17 is a graph showing the hardness distribution of the sphericalcore;

FIG. 18 is a graph showing the hardness distribution of the sphericalcore;

FIG. 19 is a graph showing the hardness distribution of the sphericalcore;

FIG. 20 is a graph showing the hardness distribution of the sphericalcore;

FIG. 21 is a graph showing the hardness distribution of the sphericalcore;

FIG. 22 is a graph showing the hardness distribution of the sphericalcore;

FIG. 23 is a graph showing the hardness distribution of the sphericalcore; and

FIG. 24 is a graph showing the relation of the 37.5% point hardness andthe compression deformation amount of the core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The golf ball of the present invention has a spherical core and at leastone cover layer covering the spherical core. The golf ball constructionof the present invention is not limited, as long as the golf ballcomprises a spherical core and at least one cover layer covering thespherical core. FIG. 1 is a partially cutaway sectional view showing thegolf ball 2 according to the preferable embodiment of the presentinvention. The golf ball 2 comprises a spherical core 4, and a cover 12covering the spherical core 4. A plurality of dimples 14 are formed onthe surface of the cover. Other portions than dimples 14 on the surfaceof the golf ball 2 are land 16. The golf ball 2 is provided with a paintlayer and a mark layer outside the cover 12, but these layers are notdepicted.

The golf ball of the present invention is characterized in that ahardness distribution of the spherical core satisfies the followingrequirements (I) to (Ill), when JIS-C hardness of the spherical core ismeasured at nine points obtained by dividing a radius of the sphericalcore into equal parts having a 12.5% interval, namely at nine pointslocated at distances of 0% (core center), 12.5%, 25.0%, 37.5%, 50.0%,62.5%, 75.0%, 87.5% and 100% (core surface) from the core center. Thespherical core having such a hardness distribution provides a loweredspin rate on driver shots, and as a result, the golf ball of the presentinvention travels a great flight distance on driver shots.

(I) A hardness difference (Hs−Ho) between a surface hardness (Hs) and acenter hardness (Ho) is 20 or more in JIS-C hardness. If the hardnessdifference between the core surface and the core center is large, a golfball traveling a great flight distance due to a higher launch angle anda lower spin rate can be obtained. The hardness difference (Hs−Ho) ispreferably 25 or more, more preferably 28 or more, and is preferably 80or less, more preferably 70 or less, even more preferably 60 or less, inJIS-C hardness.

(II) A ratio ((H_(37.5)−Ho)/(H_(75.0)−Ho))) of a hardness difference(H_(37.5)−Ho) between a 37.5% point hardness (H_(37.5)) and a centerhardness (Ho) to a hardness difference (H_(75.0)−Ho) between a 75.0%point hardness (H_(75.0)) and a center hardness (Ho) is 0.6 or less.Since the smaller the ratio of ((H_(37.5)−Ho)/(H_(75.0)−Ho)) is, therelatively smaller the 37.5% point hardness (H_(37.5)) becomes, a muchmore lowered spin rate on driver shots can be obtained. The ratio of((H_(37.5)−Ho)/(H_(75.0)−Ho)) is preferably 0.55 or less, morepreferably 0.5 or less. The lower limit of the ratio of((H_(37.5)−Ho)/(H_(75.0)−Ho)) is not limited, but 0.1 is preferable.

(III) A compression deformation amount (D(mm)) when applying a load froman initial load of 98 N to a final load of 1275 N to the spherical coreand a 37.5% point hardness (H_(37.5)) satisfy a relation ofH_(37.5)≦−9×D+95. The core satisfying the inequality has a high hardnessas a whole, but shows a 37.5% point hardness selectively beingcontrolled at a lower level.

The spherical core preferably has the center hardness Ho of 30 or more,more preferably 35 or more, even more preferably 40 or more in JIS-Chardness. If the center hardness Ho of the spherical core is less than30 in JIS-C hardness, the core becomes so soft that the resiliencethereof may be lowered. Further, the spherical core preferably has thecenter hardness Ho of 70 or less, more preferably 65 or less, even morepreferably 60 or less in JIS-C hardness. If the center hardness Hoexceeds 70 in JIS-C hardness, the core becomes so hard that the shotfeeling thereof tends to be lowered.

The spherical core preferably has the surface hardness Hs of 65 or more,more preferably 70 or more, and preferably has the surface hardness Hsof 100 or less, more preferably 95 or less in JIS-C hardness. If thesurface hardness of the spherical core is 65 or more in JIS-C hardness,the spherical core does not become excessively soft, and thus the betterresilience is obtained. Further, if the surface hardness of thespherical core is 100 or less in JIS-C hardness, the spherical core doesnot become excessively hard, and thus the better shot feeling isobtained.

The spherical core preferably has a diameter of 34.8 mm or more, morepreferably 36.8 mm or more, and even more preferably 38.8 mm or more,and preferably has a diameter of 42.2 mm or less, more preferably 41.8mm or less, and even more preferably 41.2 mm or less, and mostpreferably 40.8 mm or less. If the spherical core has the diameter of34.8 mm or more, the thickness of the cover does not become too thickand thus the resilience becomes better. On the other hand, if thespherical core has the diameter of 42.2 mm or less, the thickness of thecover does not become too thin and thus the cover functions better.

When the spherical core has a diameter of from 34.8 mm to 42.2 mm, acompression deformation amount (a shrinking amount of the spherical corealong the compression direction) of the spherical core when applying aload from an initial load of 98 N to a final load of 1275 N ispreferably 2.0 mm or more, more preferably 2.8 mm or more, and ispreferably 6.0 mm or less, more preferably 5.0 mm or less. If thecompression deformation amount is 2.0 mm or more, the shot feeling ofthe golf ball becomes better. If the compression deformation amount is6.0 mm or less, the resilience of the golf ball becomes better.

The spherical core preferably has a single layered structure. Unlike themulti-layered structure, the spherical core of the single layeredstructure does not have an energy loss at the interface of themulti-layered structure when being hit, and thus has an improvedresilience. The cover has a structure of at least one layer, forexample, a single layered structure, or a multi-layered structure of atleast two layers. The golf ball of the present invention indudes, forexample, a two-piece golf ball comprising a spherical core and a singlelayered cover disposed around the spherical core; a multi-piece golfball comprising a spherical core and at least two cover layers disposedaround the spherical core (including a three-piece golf ball); and awound golf ball comprising a spherical core, a rubber thread layer whichis formed around the spherical core, and a cover disposed over therubber thread layer. The present invention can be suitably applied toany one of the above golf ball.

The golf ball of the present invention has at least one cover layercovering the spherical core. The cover has at least one layer, forexample, a single layered cover, a two-layered cover comprising an innercover and an outer cover, and a multi-layered cover of three or morelayers.

The thickness of the cover is preferably 4.0 mm or less, more preferably3.0 mm or less, even more preferably 2.0 mm or less. If the thickness ofthe cover is 4.0 mm or less, the resilience and shot feeling of theobtained golf ball become better. The thickness of the cover ispreferably 0.3 mm or more, more preferably 0.5 mm or more, and even morepreferably 0.8 mm or more, and most preferably 1.0 mm or more. If thethickness of the cover is less than 0.3 mm, the durability and the wearresistance of the cover may deteriorate. If the cover has a plurality oflayers, it is preferred that the total thickness of the cover layersfalls within the above range.

The cover is formed from a cover composition. The slab hardness of thecover composition is preferably set in accordance with the desiredperformance of the golf ball. For example, in case of a so-calleddistance golf ball which focuses on a flight distance, the covercomposition preferably has a slab hardness of 50 or more, morepreferably 55 or more, and preferably has a slab hardness of 80 or less,more preferably 70 or less in shore D hardness. If the cover compositionhas the slab hardness of 50 or more, the obtained golf ball has a highlaunch angle and low spin rate on driver shots and iron shots, and thusthe flight distance becomes large. If the cover composition has the slabhardness of 80 or less, the golf ball excellent in durability isobtained.

Further, in case of a so-called spin golf ball which focuses oncontrollability, the cover composition preferably has a slab hardness ofless than 50, and preferably has a slab hardness of 20 or more, morepreferably 25 or more in shore D hardness. If the cover composition hasthe slab hardness of less than 50, the flight distance on driver shotscan be improved by the core of the present invention, as well as theobtained golf ball readily stops on the green due to the high spin rateon approach shots. If the cover composition has the slab hardness of 20or more, the abrasion resistance improves. In case of a plurality ofcover layers, the slab hardness of the cover composition constitutingeach layer can be identical or different, as long as the slab hardnessof each layer is within the above range.

The concave portions called “dimple” are usually formed on the surfaceof the cover. The total number of the dimples is preferably 200 or moreand 500 or less. If the total number is less than 200, the dimple effectis hardly obtained. On the other hand, if the total number exceeds 500,the dimple effect is hardly obtained because the size of the respectivedimples is small. The shape (shape in a plan view) of dimples includes,for example, without limitation, a circle, a polygonal shape such as aroughly triangular shape, a roughly quadrangular shape, a roughlypentagonal shape, a roughly hexagonal shape, and other irregular shape.The shape of the dimples is employed solely or at least two of them maybe used in combination.

When the golf ball having the cover has a diameter in a range from 40 mmto 45 mm, a compression deformation amount of the golf ball (a shrinkingamount of the golf ball in the compression direction thereof) whenapplying a load from an initial load of 98 N to a final load of 1275 Nto the golf ball is preferably 2.0 mm or more, more preferably 2.4 mm ormore, even more preferably 2.5 mm or more, most preferably 2.8 mm ormore, and is preferably 5.0 mm or less, more preferably 4.5 mm or less.If the compression deformation amount is 2.0 mm or more, the golf balldoes not become excessively hard, and thus exhibits good shot feeling.On the other hand, if the compression deformation amount is 5.0 mm orless, the resilience is enhanced.

The spherical core is preferably formed from a rubber compositioncontaining (a) a base rubber, (b) an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and/or a metal salt thereof as aco-crosslinking agent, (c) a crosslinking initiator and (d) a carboxylicacid and/or a salt thereof.

As (a) the base rubber used in the present invention, natural rubberand/or synthetic rubber can be used. For example, polybutadiene rubber,natural rubber, polyisoprene rubber, styrene polybutadiene rubber,ethylene-propylene-diene rubber (EPDM), or the like can be used. Theserubbers may be used solely or two or more of these rubbers may be usedin combination. Among them, typically preferred is thehigh-cispolybutadiene having a cis-1,4 bond in a proportion of 40 mass %or more, more preferably 80 mass % or more, even more preferably 90 mass% or more in view of its superior resilience property.

The high-cispolybutadiene preferably has a 1,2-vinyl bond in a contentof 2 mass % or less, more preferably 1.7 mass % or less, and even morepreferably 1.5 mass % or less. If the content of 1,2-vinyl bond isexcessively high, the resilience may be lowered.

The high-cispolybutadiene preferably includes one synthesized using arare-earth element catalyst. When a neodymium catalyst, which employs aneodymium compound of a lanthanum series rare-earth element compound, isused, a polybutadiene rubber having a high content of a cis-1,4 bond anda low content of a 1,2-vinyl bond is obtained with excellentpolymerization activity. Such polybutadiene rubber is particularlypreferred.

The high-cispolybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100°C.)) of 30 or more, more preferably 32 or more, even more preferably 35or more, and preferably has a Mooney viscosity (ML₁₊₄(100° C.)) of 140or less, more preferably 120 or less, even more preferably 100 or less,and most preferably 80 or less. It is noted that the Mooney viscosity(ML₁₊₄ (100° C.)) in the present invention is a value measured accordingto JISK6300 using an L rotor under the conditions of: a preheating timeof 1 minute; a rotor rotation time of 4 minutes; and a temperature of100° C.

The high-cispolybutadiene preferably has a molecular weight distributionMw/Mn (Mw: weight average molecular weight. Mn: number average molecularweight) of 2.0 or more, more preferably 2.2 or more, even morepreferably 2.4 or more, and most preferably 2.6 or more, and preferablyhas a molecular weight distribution Mw/Mn of 6.0 or less, morepreferably 5.0 or less, even more preferably 4.0 or less, and mostpreferably 3.4 or less. If the molecular weight distribution (Mw/Mn) ofthe high-cispolybutadiene is excessively low, the processability maydeteriorate. If the molecular weight distribution (Mw/Mn) of thehigh-cispolybutadiene is excessively high, the resilience may belowered. It is noted that the measurement of the molecular weightdistribution is conducted by gel permeation chromatography(“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differentialrefractometer as a detector under the conditions of column: GMHHXL(manufactured by Tosoh Corporation), column temperature: 40° C., andmobile phase: tetrahydrofuran, and calculated by converting based onpolystyrene standard.

(b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof is blended as a co-crosslinking agent inthe rubber composition and has an action of crosslinking a rubbermolecule by graft polymerization to a base rubber molecular chain. Inthe case that the rubber composition used in the present inventioncontains only the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms as the co-crosslinking agent, the rubber composition preferablyfurther contains (e) a metal compound as an essential component.Neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms with the metal compound in the rubber composition providessubstantially the same effect as using the metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Further, inthe case of using the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and the metal salt thereof in combination, (e) the metalcompound may be used as an optional component.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes,for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid,crotonic acid, and the like.

Examples of the metal constituting the metal salt of the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms include: a monovalent metalion such as sodium, potassium, lithium or the like: a divalent metal ionsuch as magnesium, calcium, zinc, barium, cadmium or the like; atrivalent metal ion such as aluminum or the like; and other metal ionsuch as tin, zirconium or the like. The above metal ion can be usedsolely or as a mixture of at least two of them. Among these metal ions,the divalent metal ion such as magnesium, calcium, zinc, barium, cadmiumor the like is preferable. Use of the divalent metal salt of theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms easilygenerates a metal crosslinking between the rubber molecules. Especially,as the divalent metal salt, zinc acrylate is preferable, because zincacrylate enhances the resilience of the resultant golf ball. Theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof may be used solely or in combination at least two ofthem.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms and/or the metal salt thereof is preferably 15 parts bymass or more, more preferably 20 parts by mass or more, and ispreferably 50 parts by mass or less, more preferably 45 parts by mass orless, even more preferably 35 parts by mass or less, with respect to 100parts by mass of (a) the base rubber. If the content of (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or themetal salt thereof is less than 15 parts by mass, the content of (c) thecrosslinking initiator which will be explained below must be increasedin order to obtain the appropriate hardness of the constituting memberformed from the rubber composition, which tends to cause the lowerresilience. On the other hand, if the content of (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereofexceeds 50 parts by mass, the constituting member formed from the rubbercomposition becomes excessively hard, which tends to cause the lowershot feeling.

(c) The crosslinking initiator is blended in order to crosslink (a) thebase rubber component. As (c) the crosslinking initiator, an organicperoxide is preferred. Specific examples of the organic peroxide includea dialkyl peroxide, a peroxy ester, a peroxy ketal, and a hydroperoxide.Specific examples of the dialkyl peroxide includedi(2-t-butylperoxyisopropyl)benzene (175.4° C.), dicumyl peroxide(175.2° C.), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (179.8° C.),t-butylcumyl peroxy (173.3° C.), di-t-hexyl peroxy (176.7° C.),di-t-butyl peroxy (185.9° C.),2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (194.3° C.) and the like.Specific examples of the peroxy ester include t-butyl peroxymaleate(167.5° C.), t-butylperoxy-3,3,5-trimethyl cyclohexanoate (166.0° C.),t-butyl peroxylaurate (159.4° C.), t-butylperoxy isopropyl monocarbonate(158.8° C.), t-hexyl peroxybenzoate (160.3° C.),2,5-dimethyl-2,5-di(benzoylperoxy)hexane (158.2° C.), t-butylperoxyacetate (159.9° C.) and t-butyl peroxybenzoate (166.8° C.).Specific examples of the peroxy ketal include1,1-di(t-hexylperoxy)-3,3,5-trimethyl cyclohexane (147.1° C.),1,1-di(t-hexylperoxy)cyclohexane (149.2° C.),1,1-di(t-butylperoxy)-2-methyl cyclohexane (142.1° C.),1,1-di(t-butylperoxy)cyclohexane (153.8° C.),2,2-di(t-butylperoxy)butane (159.9° C.),n-butyl-4,4-di(t-butylperoxy)valerate (172.5° C.), and2,2-di(4,4-di(t-butylperoxy)cyclohexyl) propane (153.8° C.). Specificexamples of the hydroperoxide include p-menthane hydroperoxide (199.5°C.) and diisopropylbenzene hydroperoxide (232.5° C.). Among them, thedialkyl peroxide and/or the peroxy ketal are preferable. These organicperoxides may be used solely or in combination at least two of them. Itis noted that the values described in the parentheses after the compoundnames of the above organic peroxides indicate one-minute half-lifetemperatures thereof.

The content of (c) the crosslinking initiator is preferably 0.2 part bymass or more, and more preferably 0.5 part by mass or more, and ispreferably 5 parts by mass or less, and more preferably 2.5 parts bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the content of (c) the crosslinking initiator is less than 0.2 partby mass, the constituting member formed from the rubber compositionbecomes so soft that the golf ball tends to have the lower resilience.If the content of (c) the crosslinking initiator exceeds 5 parts bymass, the amount of (b) the co-crosslinking agent must be decreased inorder to obtain the appropriate hardness of the constituting memberformed from the rubber composition, probably resulting in theinsufficient resilience or lower durability of the golf ball.

(d) The carboxylic acid and/or the salt thereof used in the presentinvention will be described. It is considered that (d) the carboxylicacid and/or the salt thereof has an action of breaking the metalcrosslinking by the metal salt of (b) the α,β-unsaturated carboxylicacid having 3 to 8 carbon atoms, in the center part of the core, whenmolding the core. (d) The carboxylic acid and/or the salt thereofincludes an aliphatic carboxylic acid and/or a salt thereof, or anaromatic carboxylic acid and/or a salt thereof. (d) The carboxylic acidand/or the salt thereof may be used alone or as a mixture of at leasttwo of them. It is noted that (d) the carboxylic acid and/or the saltthereof does not include (b) the α,β-unsaturated carboxylic acid having3 to 8 carbon atoms and/or the metal salt thereof as the co-crosslinkingagent.

The aliphatic carboxylic acid preferably indudes an aliphatic carboxylicacid having 1 to 30 carbon atoms, more preferably an aliphaticcarboxylic acid having 1 to 18 carbon atoms, even more preferably analiphatic carboxylic acid having 1 to 13 carbon atoms. It is noted that(d) the aliphatic carboxylic acid and/or the salt thereof does notinclude (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms and/or the metal salt thereof used as the co-crosslinking agent.

The aliphatic carboxylic acid may be either a saturated fatty acid or anunsaturated fatty acid. The aliphatic carboxylic acid may have abranched structure or a cyclic structure. Specific examples of thesaturated fatty acids (IUPAC name) are methanoic acid (C1), ethanoicacid (C2), propanoic acid (C3), butanoic acid (C4), pentanoic acid (C5),hexanoic acid (06), heptanoic acid (C7), octanoic acid (C8), nonanoicacid (C9), decanoic acid (C10), undecanoic acid (C11), dodecanoic acid(C12), tridecanoic acid (C13), tetradecanoic acid (C14), pentadecanoicacid (015), hexadecanoic acid (016), heptadecanoic acid (C17),octadecanoic acid (C18), nonadecanoic acid (C19), icosanoic acid (C20),henicosanoic acid (C21), docosanoic acid (C22), tricosanoic acid (C23),tetracosanoic acid (C24), pentacosanoic acid (C25), hexacosanoic acid(C26), heptacosanoic acid (027), octacosanoic acid (028), nonacosanoicacid (C29), and triacontanoic acid (C30).

Specific examples of the unsaturated fatty acid (IUPAC name) areethenoic acid (C2), propenoic acid (03), butenoic acid (C4), pentenoicacid (C5), hexenoic acid (C6), heptenoic acid (07), octenoic acid (08),nonenoic acid (C9), decenoic acid (010), undecenoic acid (C11),dodecenoic acid (C12), tridecenoic acid (C13), tetradecenoic acid (014),pentadecenoic acid (C15), hexadecenoic acid (C16), heptadecenoic acid(017), octadecenoic acid (C18), nonadecenoic acid (C19), icosenoic acid(020), henicosenoic acid (021), docosenoic acid (022), tricosenoic acid(C23), tetracosenoic acid (C24), pentacosenoic acid (025), hexacosenoicacid (C26), heptacosenoic acid (C27), octacosenoic acid (C28),nonacosenoic acid (C29), and triacontenoic acid (C30).

Specific examples of the fatty acid (common name) are, formic acid (C1),acetic acid (C2), propionic acid (C3), butyric acid (C4), valeric acid(C5), caproic acid (C6), enanthic acid (C7), caprylic acid (C8),pelargonic acid (C9), capric acid (C10), lauric acid (C12), myristicacid (C14), myristoleic acid (C14), pentadecylic acid (C15), palmiticacid (C16), palmitoleic acid (C16), margaric acid (C17), stearic acid(C18), elaidic acid (C18), vaccenic acid (C18), oleic acid (C18),linoleic acid (C18), linolenic acid (018), 12-hydroxystearic acid (C18),arachidic acid (C20), gadoleic acid (C20), arachidonic acid (C20),eicosenoic acid (C20), behenic acid (C22), erucic acid (C22), lignocericacid (C24), nervonic acid (C24), cerotic acid (C26), montanic acid(C28), and melissic acid (C30).

The aromatic carboxylic acid includes an aromatic carboxylic acid havinga benzene ring in the molecule thereof, and an aromatic carboxylic acidhaving a heteroaromatic ring in the molecule thereof. The aromaticcarboxylic acid may be used solely or in combination of at least two ofthem.

Specific examples of (d) the carboxylic acid having a benzene ringinclude, for example, an aromatic carboxylic acid having a carboxylgroup directly bonding to the benzene ring, an aromatic-aliphaticcarboxylic acid having an aliphatic carboxylic acid bonding to thebenzene ring, a polynuclear aromatic carboxylic acid having a carboxylgroup directly bonding to a fused benzene ring, and a polynucleararomatic-aliphatic carboxylic acid having an aliphatic carboxylic acidbonding to a fused benzene ring. The fused benzene ring structureincludes, for example, naphthalene, anthracene, phenalene, phenanthrene,tetracene and pyrene.

The number of carboxyl group in (d) the carboxylic acid having a benzenering may be one (monocarboxylic acid), two or more (polycarboxylicacid), but one is preferred. A substituent group other than a carboxylgroup may directly bond to the benzene ring or fused benzene ring. Sucha substituent group includes, for example, an alkyl group (preferably analkyl group having 1 to 4 carbon atoms), an aryl group (preferablyphenyl group), an amino group, a hydroxyl group, an alkoxy group(preferably an alkoxy group having 1 to 4 carbon atoms), an oxo group,or a halogen group.

Specific examples of the aromatic carboxylic acid having a carboxylgroup directly bonding to the benzene ring include, for example, benzoicacid (C7), phthalic acid (C8), isophthalic acid (C8), terephthalic acid(C8), benzene-1,2,3-tricarboxylic acid (C9), benzene-1,2,4-tricarboxylicacid (C9), benzene-1,3,5-tricarboxylic acid (C9),benzene-1,2,3,4-tetracarboxylic acid (010),benzene-1,2,3,5-tetracarboxylic acid (C10),benzene-1,2,4,5-tetracarboxylic acid (C10), and benzene hexacarboxylicacid (C12). Specific examples of the aromatic-aliphatic carboxylic acidhaving an aliphatic carboxylic acid bonding to the benzene ring include,for example, phenylacetic acid (C8), 2-phenylpropanoic acid (C9), and3-phenylpropanoic acid (C9).

Furthermore, examples of the carboxylic acid having a benzene ringsubstituted with an alkyl group, aryl group, amino group, hydroxylgroup, alkoxy group, or oxo group include, for example, methylbenzoicacid (C8), dimethylbenzoic acid (C9), 2,3,4-trimethylbenzoic acid (C10),2,3,5-trimethylbenzoic acid (C10), 2,4,5-trimethylbenzoic acid (C10),2,4,6-trimethylbenzoic acid (C10), 3,4,5-trimethylbenzoic acid (C10),4-isopropylbenzoic acid (C10), 4-tert-butylbenzoic acid (C11),5-methylisophthalic acid (C9), biphenyl-4-carboxylic acid (C13),biphenyl-2,2′-dicarboxylic acid (C14), 4-dimethylaminobenzoic acid (C9),2-hydroxybenzoic acid (C7), methoxybenzoic acid (C8),hydroxy(methyl)benzoic acid (C8), 2-hydroxy-3-methylbenzoic acid (C8),2-hydroxy-4-methylbenzoic acid (C8), 2-hydroxy-5-methylbenzoic acid(C8), 2,3-dihydroxybenzoic acid (C7), 2,4-dihydroxybenzoic acid (C7),2,6-dihydroxybenzoic acid (C7), 3,4-dihydroxybenzoic acid (C7),3,5-dihydroxybenzoic acid (C7), 4-hydroxy-3-methoxybenzoic acid (C8),3-hydroxy-4-methoxybenzoic acid (C8), 3,4-dimethoxybenzoic acid (C9),2,3-dimethoxybenzoic acid (C9), 2,4-dimethoxybenzoic acid (C9),2,4-dihydroxy-6-methylbenzoic acid (C8), 4,5-dimethoxyphthalic acid(C10), 3,4,5-trihydroxybenzoic acid (C7), 4-hydroxy-3,5-dimethoxybenzoicacid (C9), 2,4,5-trimethoxybenzoic acid (C10), hydroxy(phenyl)aceticacid (C8), hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid (C9),(4-methoxyphenyl)acetic acid (C9), (2,5-dihydroxyphenyl)acetic acid(C8), (3,4-dihydroxyphenyl)acetic acid (C8),(4-hydroxy-3-methoxyphenyl)acetic acid (C9),(3-hydroxy-4-methoxyphenyl)acetic acid (C9), (3,4-dimethoxyphenyl)aceticacid (C10), (2,3-dimethoxyphenyl)acetic acid (C10),2-(carboxymethyl)benzoic acid (C9), 3-(carboxymethyl)benzoic acid (C9),4-(carboxymethyl)benzoic acid (C9), 2-(carboxycarbonyl)benzoic acid(C9), 3-(carboxycarbonyl)benzoic acid (C9), 4-(carboxycarbonyl)benzoicacid (C9), 2-hydroxy-2-phenylpropanoic acid (C9),3-hydroxy-2-phenylpropanoic acid (C9), 3-(2-hydroxyphenyl)propanoic acid(C9), 3-(4-hydroxyphenyl)propanoic acid (C9),3-(3,4-dihydroxyphenyl)propanoic acid (C9),3-(4-hydroxy-3-methoxyphenyl)propanoic acid (C10),3-(3-hydroxy-4-methoxyphenyl)propanoic acid (C10),3-(4-hydroxyphenyl)acrylic acid (C9), 3-(2,4-dihydroxyphenyl)acrylicacid (C9), 3-(3,4-dihydroxyphenyl)acrylic acid (C9),3-(4-hydroxy-3-methoxyphenyl)acrylic acid (C10),3-(3-hydroxy-4-methoxyphenyl) acrylic acid (C10), and3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic add) (C11).

The carboxylic acid having the benzene ring substituted with halogenincludes, for example, carboxylic acids where at least one hydrogen ofbenzoic add is substituted with a fluoro group such as fluorobenzoicadd, difluorobenzoic acid, trifluorobenzoic acid, tetrafluorobenzoicacid, and pentafluorobenzoic acid; carboxylic acids where at least onehydrogen of benzoic acid is substituted with a chloro group such aschlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid,tetrachlorobenzoic acid, and pentachlorobenzoic acid; carboxylic acidswhere at least one hydrogen of benzoic acid is substituted with a bromogroup such as bromobenzoic acid, dibromobenzoic acid, tribromobenzoicacid, tetrabromobenzoic acid, and pentabromobenzoic acid; and carboxylicacids where at least one hydrogen of benzoic acid is substituted with aiodo group such as iodobenzoic acid, diiodobenzoic acid, triiodobenzoicacid, tetraiodobenzoic acid, and pentaiodobenzoic acid.

Specific examples of the polynuclear aromatic carboxylic acid having acarboxyl group directly bonding to the fused benzene ring include1-naphthalene carboxylic acid, 2-naphthalene carboxylic acid,1-anthracene carboxylic acid, 2-anthracene carboxylic acid, 9-anthracenecarboxylic acid, phenanthrene carboxylic acid, and pyrene carboxylicacid. Specific examples of the polynuclear aromatic-aliphatic carboxylicacid where the aliphatic carboxylic acid is bonded to the fused benzenering include naphthylacetic acid, and naphthylpropionic acid.

The carboxylic acid having a fused benzene ring substituted with halogenincludes, for example, fluoronaphthalene carboxylic acid,chloronaphthalene carboxylic acid, bromonaphthalene carboxylic acid,fluoroanthracene carboxylic acid, chloroanthracene carboxylic acid, andbromoanthracene carboxylic acid.

The carboxylic acid having a heteroaromatic ring includes, for example,a carboxylic acid where a carboxylic acid is directly bonded to theheteroaromatic ring. The hetero atom in the heteroaromatic ring can beone kind or two or more kinds. The hetero atom includes a nitrogen atom,oxygen atom, sulfur atom or the like. Among them, the oxygen atom orsulfur atom is preferred. The number of the hetero atom in theheteroaromatic ring is not particularly limited, but preferably 2 orless, and more preferably 1. The heteroaromatic ring includes, forexample, a pyrrole ring, furan ring, thiophene ring, imidazole ring,pyrazole ring, oxazole ring, thiazole ring, pyridine ring, pyrazinering, indole ring, quinolone ring, benzofuran ring, and benzothiophenering.

(d) The carboxylic acid having a heteroaromatic ring may be a compoundhaving only a carboxyl group as a substituent group to theheteroaromatic ring, or a compound having another substituent groupdirectly bonding to the heteroaromatic ring in addition to the carboxylgroup. Further, the substituent group may bond to a nitrogen atomconstituting the heteroaromatic ring. The substituent group includes,for example, halogen, a hydroxyl group, a mercapto group, an alkylgroup, an aryl group, an aralkyl group, an alkylaryl group, an alkoxylgroup, an amino group which may be substituted, a cyano group, or athiocarboxyl group.

Specific examples of the carboxylic acid having the heteroaromatic ringand/or the salt thereof include, carboxylic acids having a five-memberedheteroaromatic ring such as a pyrrole carboxylic acid, furan carboxylicacid, thiophene carboxylic acid, imidazole carboxylic acid, pyrazolecarboxylic acid, oxazole carboxylic acid, and thiazole carboxylic acid;carboxylic acids having a six-membered heteroaromatic ring such as apyridine carboxylic acid, and a pyrazine carboxylic acid; and carboxylicacids having a fused heteroaromatic ring such as an indolecarboxylicacid, quinolinecarboxylic acid, benzofuran carboxylic acid, andbenzothiophene carboxylic acid.

As (d) the salt of the aliphatic carboxylic acid or aromatic carboxylicacid, a salt of the aliphatic carboxylic acid or aromatic carboxylicacid described above may be used. The cation component of the salt ofthese carboxylic acids may be any one of a metal ion, an ammonium ionand an organic cation. The metal ion includes monovalent metal ions suchas sodium, potassium, lithium, silver and the like; divalent metal ionssuch as magnesium, calcium, zinc, barium, cadmium, copper, cobalt,nickel, manganese and the like; trivalent metal ions such as aluminum,iron and the like; and other ions such as tin, zirconium, titanium andthe like. The cation components may be used alone or as a mixture of atleast two of them.

The organic cation includes a cation having a carbon chain. The organiccation includes, for example, without limitation, an organic ammoniumion. Examples of the organic ammonium ion are: primary ammonium ionssuch as stearyl ammonium ion, hexyl ammonium ion, octyl ammonium ion,2-ethyl hexyl ammonium ion or the like; secondary ammonium ions such asdodecyl(lauryl) ammonium ion, octadecyl(stearyl) ammonium ion or thelike; tertiary ammonium ions such as trioctyl ammonium ion or the like;and quaternary ammonium ions such as dioctyldimethyl ammonium ion,distearyldimethyl ammonium ion or the like. These organic cation may beused alone or as a mixture of at least two of them.

(d) The aliphatic carboxylic acid and/or the salt thereof preferablyincludes a saturated fatty acid and/or the salt thereof. Preferableexamples thereof include caprylic acid (octanoic acid), pelargonic acid(nonanoic acid), capric acid (decanoic acid), lauric acid, myristicacid, palmitic acid, stearic acid, behenic acid, and oleic add, or apotassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt,iron salt, copper salt, nickel salt, or cobalt salt of the abovealiphatic carboxylic acids. (d) The aromatic carboxylic acid and/or thesalt thereof preferably includes benzoic acid, butylbenzoic acid, anisicacid (methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoicacid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoicacid, trichlorobenzoic acid, acetoxybenzoic acid, biphenyl carboxylicacid, naphthalene carboxylic acid, anthracene carboxylic acid, furancarboxylic acid, and thenoyl carboxylic acid, or a potassium salt,magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt,copper salt, nickel salt, or cobalt salt of the above aromaticcarboxylic acids.

The content of (d) the carboxylic acid and/or the salt thereof ispreferably 0.5 part by mass or more, more preferably 1.0 parts by massor more, even more preferably 1.5 parts by mass or more, and ispreferably 30 parts by mass or less, more preferably 20 parts by mass orless, even more preferably 15 parts by mass or less, with respect to 100parts by mass of (a) the base rubber. If the content of (d) thecarboxylic acid and/or the salt thereof is excessively low, the effectof adding (d) the carboxylic acid andior the salt thereof is notsufficient, and thus the degree of the outer-hard inner-soft structureof the spherical core may be lowered. If the content is excessivelyhigh, the resilience of the core may be lowered, since the hardness ofthe resultant core may be lowered as a whole.

There are cases where the surface of the zinc acrylate used as theco-crosslinking agent is treated with zinc stearate to improve thedispersibility to the rubber. In the case of using zinc acrylate whosesurface is treated with zinc stearate, in the present invention, theamount of zinc stearate used as a surface treating agent is included inthe content of (d) the carboxylic acid and/or the salt thereof. Forexample, if 25 parts by mass of zinc acrylate whose surface treatmentamount with zinc stearate is 10 mass % is used, the amount of zincstearate is 2.5 parts by mass and the amount of zinc acrylate is 22.5parts by mass. Thus, 2.5 parts by mass is counted as the content of (d)the carboxylic acid and/or the salt thereof.

In the case that the rubber composition used in the present inventioncontains only the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms as the co-crosslinking agent, the rubber composition furthercontains (e) the metal compound as an essential component. (e) The metalcompound is not limited as long as it can neutralize (b) theα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubbercomposition. (e) The metal compound includes, for example, a metalhydroxide such as magnesium hydroxide, zinc hydroxide, calciumhydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide,copper hydroxide, or the like; a metal oxide such as magnesium oxide,calcium oxide, zinc oxide, copper oxide, or the like; and a metalcarbonate such as magnesium carbonate, zinc carbonate, calciumcarbonate, sodium carbonate, lithium carbonate, potassium carbonate, orthe like. Among these, (e) the metal compound preferably includes adivalent metal compound, more preferably includes a zinc compound. Thedivalent metal compound reacts with the α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms, thereby forming a metal crosslinking. Use ofthe zinc compound provides a golf ball with excellent resilience. (e)The metal compound can be used solely or as a mixture of at least two ofthem. The content of (e) the metal compound may be appropriatelydetermined in accordance with the desired degree of neutralization of(b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition used in the present invention preferably furthercontains (f) an organic sulfur compound. By using (c) the crosslinkinginitiator, (d) the carboxylic acid and/or the salt thereof and (f) theorganic sulfur compound in combination for the rubber composition, thedegree of the outer-hard and inner-soft structure of the core can becontrolled to a higher extent.

(f) The organic sulfur compound is not particularly limited, as long asit is an organic compound having a sulfur atom in the molecule thereof.Examples thereof include an organic compound having a thiol group (—SH)or a polysulfide bond having 2 to 4 sulfur atoms (-S-S-, -S-S-S-, or-S-S-S-S-), and a metal salt thereof (-SM, -S-M-S-, -S-M-S-S-,-S-S-M-S-S-, -S-M-S-S-S-, or the like; M is a metal atom). Examples ofthe metal salt include a monovalent metal salt such as sodium, lithium,potassium, copper (I), and silver (I) or the like, and a divalent metalsalt such as zinc, magnesium, calcium, strontium, barium, titanium (II),manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), tin(II) or the like. Furthermore, (f) the organic sulfur compound may beany one of an aliphatic compound (aliphatic thiol, aliphaticthiocarboxylic acid, aliphatic dithiocarboxylic acid, aliphaticpolysulfide, or the like), heterocyclic compound, alicyclic compound(alicyclic thiol, alicyclic thiocarboxylic acid, alicyclicdithiocarboxylic acid, alicyclic polysulfide, or the like), and aromaticcompound.

(f) The organic sulfur compound includes, for example, thiols(thiophenols and thionaphthols), polysulfides, thiurams, thiocarboxylicacids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, andthiazoles.

Examples of the thiols include, for example, thiophenols andthionaphthols. The thiophenols include, for example, thiophenol;thiophenols substituted with a fluoro group, such as 4-fluorothiophenol,2,5-difluorothiophenol, 2,6-difluorothiophenol2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol,pentafluorothiophenol and the like; thiophenols substituted with achloro group, such as 2-chlorothiophenol, 4-chlorothiophenol,2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol,pentachlorothiophenol and the like; thiophenols substituted with a bromogroup, such as 4-bromothiophenol, 2,5-dibromothiophenol,2,6-dibromothiophenol, 2,4,5-tribromothiophenol,2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like;thiophenols substituted with an iodo group, such as 4-iodothiophenol,2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol,2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like; or ametal salt thereof. As the metal salt, a zinc salt is preferred.

Examples of the thionaphthols (naphthalenethiols) are 2-thionaphthol,1-thionaphthol, 1-chloro-2-thionaphthol 2-chloro-1-thionaphthol,1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol,2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol 2-cyano-1-thionaphthol,1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and a metal saltthereof. Preferable examples include 2-thionaphthol, 1-thionaphthol, orthe metal salt thereof. The metal salt is preferably a divalent metalsalt, more preferably a zinc salt. Specific examples of the metal saltinclude, for example, the zinc salt of 1-thionaphthol and the zinc saltof 2-thionaphthol.

The polysulfides are organic sulfur compounds having a polysulfide bond,and include, for example, disulfides, trisulfides, and tetrasulfides.The polysulfides preferably include diphenylpolysulfides.

Examples of the diphenylpolysulfides include: diphenyldisulfide;diphenyldisulfides substituted with a halogen group, such asbis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide,bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide,bis(2,4,5-trichlorophenyl)disulfide,bis(2,4,5,6-tetrachlorophenyl)disulfide,bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,4,5-tribromophenyl)disulfide,bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide,bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide,bis(2,4,5,6-tetraiodophenyl)disulfide, bis(pentaiodophenyl)disulfide;diphenyldisulfides substituted with an alkyl group, such asbis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide,bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide,bis(2,4,5-tri-t-butylphenyl)disulfide,bis(penta-t-butylphenyl)disulfide; and the like.

The thiurams include, for example, thiurammonosulfides such astetramethylthiurammonosulfide; thiuramdisulfides such astetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide; and thiuramtetrasulfides such asdipentamethylenethiuramtetrasulfide. The thiocarboxylic acids include,for example, a naphthalenethiocarboxylic acid. The dithiocarboxylicacids include, for example, a naphthalenedithiocarboxylic acid. Thesulfenamides include, for example, N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide.

(f) The organic sulfur compound preferably includes thiophenols and/orthe metal salt thereof, thionaphthols and/or the metal salt thereof,diphenyldisulfides and thiuramdisulfides, and more preferably2,4-dichlorothiophenol, 2,6-difluorothiophenol, 2,6-dichlorothiophenol,2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol,pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol,diphenyldisulfide, bis(2,6-difluorophenyl)disulfide,bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide,bis(2,6-diiodophenyl)disulfide, and bis(pentabromophenyl)disulfide.

The content of (f) the organic sulfur compound is preferably 0.05 partby mass or more, more preferably 0.1 part by mass or more, and ispreferably 5.0 parts by mass or less, more preferably 2.0 parts by massor less, with respect to 100 parts by mass of (a) the base rubber. Ifthe content of (f) the organic sulfur compound is less than 0.05 part bymass, the effect of adding (f) the organic sulfur compound cannot beobtained and thus the resilience of the golf ball may not be enhanced.If the content of (f) the organic sulfur compound exceeds 5.0 parts bymass, the compression deformation amount of the obtained golf ballbecomes large and thus the resilience may be lowered.

The rubber composition used in the present invention may include anadditive such as a pigment, a filler for adjusting weight or the like,an antioxidant, a peptizing agent, and a softener where necessary.Examples of the pigment blended in the rubber composition include awhite pigment, a blue pigment, and a purple pigment.

As the white pigment, titanium oxide is preferably used. The type oftitanium oxide is not particularly limited, but rutile type ispreferably used because of the high opacity. The blending amount oftitanium oxide is preferably 0.5 part by mass or more, and morepreferably 2 parts by mass or more, and is preferably 8 parts by mass orless, and more preferably 5 parts by mass or less, with respect to 100parts by mass of (a) the base rubber.

It is also preferred that the rubber composition contains both a whitepigment and a blue pigment. The blue pigment is blended in order tocause white color to be vivid, and examples thereof include ultramarineblue, cobalt blue, and phthalocyanine blue. Examples of the purplepigment include anthraquinone violet, dioxazine violet, and methylviolet.

The blending amount of the blue pigment is preferably 0.001 part by massor more, and more preferably 0.05 part by mass or more, and ispreferably 0.2 part by mass or less, and more preferably 0.1 part bymass or less, with respect to 100 parts by mass of (a) the base rubber.If the blending amount of the blue pigment is less than 0.001 part bymass, blueness is insufficient, and the color looks yellowish. If theblending amount of the blue pigment exceeds 0.2 part by mass, bluenessis excessively strong, and a vivid white appearance is not provided.

The filler blended in the rubber composition is used as a weightadjusting agent for mainly adjusting the weight of the golf ballobtained as a final product. The filler may be blended where necessary.The filler includes, for example, an inorganic filler such as zincoxide, barium sulfate, calcium carbonate, magnesium oxide, tungstenpowder, molybdenum powder, or the like. Zinc oxide is preferably used asthe filler. It is considered that zinc oxide functions as avulcanization activator and increases the hardness of the entire core.The content of the filler is preferably 0.5 part by mass or more, morepreferably 1 part by mass or more, and is preferably 30 parts by mass orless, more preferably 25 parts by mass or less, even more preferably 20parts by mass or less, with respect to 100 parts by mass of the baserubber. If the content of the filler is less than 0.5 part by mass, itis difficult to adjust the weight, while if the content of the fillerexceeds 30 parts by mass, the weight ratio of the rubber component isreduced and thus the resilience tends to be lowered.

The blending amount of the antioxidant is preferably 0.1 part by mass ormore and 1 part by mass or less, with respect to 100 parts by mass of(a) the base rubber. In addition, the blending amount of the peptizingagent is preferably 0.1 part by mass or more and 5 parts by mass orless, with respect to 100 parts by mass of (a) the base rubber.

The cover composition used in the present invention contains a resincomponent. Examples of the resin component include, for example, anionomer resin; a thermoplastic polyurethane elastomer having acommercial name of “Elastollan (registered trademark)” commerciallyavailable from BASF Japan Ltd; a thermoplastic polyamide elastomerhaving a commercial name of “Pebax (registered trademark)” commerciallyavailable from Arkema K. K.; a thermoplastic polyester elastomer havinga commercial name of “Hytrel (registered trademark)” commerciallyavailable from Du Pont-Toray Co., Ltd.; and a thermoplastic styreneelastomer having a commercial name of “Rabalon (registered trademark)”commercially available from Mitsubishi Chemical Corporation; and thelike. The ionomer resin includes a product prepared by neutralizing atleast a part of carboxyl groups in a binary copolymer composed of anolefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atomswith a metal ion, a product prepared by neutralizing at least a part ofcarboxyl groups in a ternary copolymer composed of an olefin, anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and anα,β-unsaturated carboxylic acid ester with a metal ion, or a mixture ofthose. The olefin preferably includes an olefin having 2 to 8 carbonatoms. Examples of the olefin are ethylene, propylene, butene, pentene,hexene, heptene, and octene. The olefin more preferably includesethylene. Examples of the α,β-unsaturated carboxylic add having 3 to 8carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleicacid and crotonic acid. Among these, acrylic acid and methacrylic acidare particularly preferred. Examples of the α,β-unsaturated carboxylicacid ester include methyl ester, ethyl ester, propyl ester, n-butylester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid,maleic add or the like. In particular, acrylic acid ester andmethacrylic acid ester are preferable. Among these, the ionomer resinpreferably includes a metal ion-neutralized product of a binarycopolymer composed of ethylene-(meth)acrylic acid and a metalion-neutralized product of a ternary copolymer composed ofethylene-(meth)acrylic acid-(meth)acrylic acid ester.

Specific examples of the ionomer resin include trade name “Himilan(registered trademark) (e.g. the binary copolymerized ionomer such asHimilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706(Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternarycopolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))”commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples of the ionomer resin also include “Surlyn (registeredtrademark) (e.g. the binary copolymerized ionomer such as Surlyn 8945(Na), Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120(Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930(Li), Surlyn 7940 (Li), Surlyn AD8546 (Li); and the ternarycopolymerized ionomer such as Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn9320 (Zn), Surlyn 6320 (Mg), HPF 1000 (Mg), HPF 2000 (Mg)” commerciallyavailable from E.I. du Pont de Nemours and Company.

Further, examples of the ionomer resin also include Iotek (registeredtrademark) (e.g. the binary copolymerized ionomer such as Iotek 8000(Na), Iotek 8030 (Na), Iotek 7010 (Zn), Iotek 7030 (Zn); and the ternarycopolymerized ionomer such as Iotek 7510 (Zn), Iotek 7520 (Zn))commercially available from ExxonMobil Chemical Corporation.

It is noted that Na, Zn. Li, and Mg described in the parentheses afterthe trade names indicate metal ion type for neutralizing the ionomerresin. The ionomer resin may be used solely or in combination at leasttwo of them.

The cover composition preferably includes, as a resin component, athermoplastic polyurethane elastomer or an ionomer resin. In case ofusing the ionomer resin, it is preferred to use a thermoplastic styreneelastomer together. The content of the polyurethane or ionomer resin inthe resin component of the cover composition is preferably 50 mass % ormore, more preferably 60 mass % or more, and even more preferably 70mass % or more.

In the present invention, the cover composition may further contain apigment component such as a white pigment (for example, titanium oxide),a blue pigment, and a red pigment; a weight adjusting agent such as zincoxide, calcium carbonate, and barium sulfate; a dispersant; anantioxidant; an ultraviolet absorber; a light stabilizer; a fluorescentmaterial or a fluorescent brightener; and the like, as long as they donot impair the performance of the cover.

The content of the white pigment (for example, titanium oxide) ispreferably 0.5 part by mass or more, more preferably 1 part by mass ormore, and is preferably 10 parts by mass or less, more preferably 8parts by mass or less, with respect to 100 parts by mass of the resincomponent constituting the cover. If the content of the white pigment is0.5 part by mass or more, it is possible to impart the opacity to theresultant cover. Further, if the content of the white pigment is morethan 10 parts by mass, the durability of the resultant cover maydeteriorate.

The method for manufacturing the golf ball of the present invention willbe explained below.

The rubber composition used in the present invention is obtained bymixing and kneading (a) the base rubber, (b) the α,β-unsaturatedcarboxylic acid having 3 to 8 carbon atoms andtor the metal saltthereof, (c) the crosslinking initiator, and (d) the carboxylic acidandtor the salt thereof, and other additives where necessary. Thekneading can be conducted, without any limitation, with a well-knownkneading machine such as a kneading roll, a banbury mixer, a kneader, orthe like.

The spherical core of the golf ball of the present invention can beobtained by heat pressing the rubber composition after kneaded. Thetemperature for heat pressing is preferably 120° C. or more, morepreferably 130° C. or more, and is preferably 170° C. or less. If themolding temperature exceeds 170° C., the surface hardness of the coretends to decrease. The molding pressure preferably ranges from 2.9 MPato 11.8 MPa. The molding time preferably ranges from 10 minutes to 60minutes.

The heat press temperature is preferably t-40° C. or more, morepreferably t-38° C. or more, even more preferably t-36° C. or more, andis preferably t-15° C. or less, more preferably t-17° C. or less, evenmore preferably t-19° C. or less, wherein t ° C. is defined as theone-minute half-life temperature of (c) the crosslinking initiator. Ifthe heat press temperature is t-40° C. or more, the degree of theouter-hard and inner-soft structure of the core can be enhanced, and ifthe heat press temperature is t-15° C. or less, the core having thehardness distribution prescribed in the present invention can beobtained, thus the effect of lowering the spin rate on driver shots isenhanced. In the case of blending two or more kinds of (c) thecrosslinking initiator in the rubber composition, the heat presstemperature may be adjusted to satisfy the above range relative toone-minute half-life temperatures of ail of (c) the crosslinkinginitiators.

An embodiment for molding the cover is not particularly limited, andincludes an embodiment which comprises injection-molding the covercomposition directly onto the core, or an embodiment which comprisesmolding the cover composition into a hollow-shell, covering the corewith a plurality of the hollow-shells and subjecting the core with aplurality of the hollow shells to the compression-molding (preferably anembodiment which comprises molding the cover composition into a halfhollow-shell, covering the core with the two half hollow-shells, andsubjecting the core with the two half hollow-shells to thecompression-molding).

When molding the cover in a compression-molding method, molding of thehalf shell can be performed by either compression-molding method orinjection-molding method, but the compression-molding method ispreferred. The compression-molding of the cover composition into halfshell can be carried out, for example, under a pressure of 1 MPa or moreand 20 MPa or less at a temperature of −20° C. or more and 70° C. orless relative to the flow beginning temperature of the covercomposition. By performing the molding under the above conditions, ahalf shell having a uniform thickness can be formed. Examples of amethod for molding the cover using half shells include a method ofcovering the core with two half shells and then subjecting the core withthe two half shells to the compression-molding. The compression-moldingof half shells into the cover can be carried out, for example, under apressure of 0.5 MPa or more and 25 MPa or less at a temperature of −20°C. or more and 70° C. or less relative to the flow beginning temperatureof the cover composition. By performing the molding under the aboveconditions, a golf ball cover having a uniform thickness can be formed.

In the case of injection-molding the cover composition, the covercomposition extruded in a pellet form beforehand may be used forinjection-molding, or the materials such as the base resin componentsand the pigment may be dry blended, followed by directlyinjection-molding the blended material. It is preferred to use upper andlower molds having a spherical cavity and pimples for forming the cover,wherein a part of the pimples also serves as a retractable hold pin.When molding the cover by injection-molding, the hold pin is protrudedto hold the core, and the cover composition which has been heated andmelted is charged and then cooled to obtain a cover. For example, it ispreferred that the cover composition heated and melted at thetemperature ranging from 200° C. to 250° C. is charged into a mold heldunder the pressure of 9 MPa to 15 MPa for 0.5 to 5 seconds, and aftercooling for 10 to 60 seconds, the mold is opened and the golf ball withthe cover molded is ejected from the mold.

After the cover is molded, the mold is opened and the golf ball body isejected from the mold, and as necessary, the golf ball body ispreferably subjected to surface treatments such as deburring, cleaning,and sandblast. If desired, a paint film or a mark may be formed. Thepaint film preferably has a thickness of, but not limited to, 5 μm orlarger, and more preferably 7 μm or larger, and preferably has athickness of 50 μm or smaller, and more preferably 40 μm or smaller,even more preferably 30 μm or smaller. If the thickness is smaller than5 μm, the paint film is easy to wear off due to continued use of thegolf ball, and if the thickness is larger than 50 μm, the effect of thedimples is reduced, resulting in lowering flying performance of the golfball.

EXAMPLES

Hereinafter, the present invention will be described in detail by way ofexample. The present invention is not limited to examples describedbelow. Various changes and modifications can be made without departingfrom the spirit and scope of the present invention.

[Evaluation Methods] (1) Compression Deformation Amount (mm)

A compression deformation amount of the core or golf ball (a shrinkingamount of the core or golf ball in the compression direction thereof),when applying a load from an initial load of 98 N to a final load of1275 N to the core or golf ball, was measured.

(2) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection-moldingthe cover composition, and stored at 23° C. for two weeks. Three or moreof these sheets were stacked on one another so as not to be affected bythe measuring substrate on which the sheets were placed, and thehardness of the stack was measured with a type P1 auto loading durometermanufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D typespring hardness tester prescribed in ASTM-02240.

(3) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co.,Ltd., provided with a JIS-C type spring hardness tester was used tomeasure the hardness of the spherical core. The hardness measured at thesurface of the spherical core was adopted as the surface hardness of thespherical core. The spherical core was cut into two hemispheres toobtain a cut plane, and the hardness was measured at the central pointand at predetermined distances from the central point. The core hardnesswas measured at 4 points at predetermined distances from the centralpoint of the cut plane of the core. The core hardness was calculated byaveraging the hardness measured at the 4 points.

(4) Flight Distance (m) and Spin Rate (rpm) on Driver Shots

A metal-headed W#1 driver (XXIO, Shaft: S, loft: 11°, manufactured byDunlop Sports Limited) was installed on a swing robot M/C manufacturedby Golf Laboratories, Inc. A golf ball was hit at a head speed of 40m/sec, and the spin rate right after hitting the golf ball and theflight distance (the distance from the launch point to the stop point)were measured. This measurement was conducted twelve times for each golfball, and the average value thereof was adopted as the measurement valuefor the golf ball. A sequence of photographs of the hit golf ball weretaken for measuring the spin rate (rpm) right after hitting the golfball. The flight distance and spin rate of the golf ball on driver shotsare shown as a difference from those of the golf ball No. 5 for the golfballs No. 1 to No. 4, No. 6 to No. 9, and are shown as a difference fromthose of the golf ball No. 21 for the golf balls No. 10 to No. 20, andNo. 22.

[Production of Golf Ball] (1) Production of Core

The rubber compositions having formulations shown in Tables 1 to 3 werekneaded with a kneading roll and extruded with an extruder to prepareplugs. The obtained plugs were heat pressed in upper and lower molds,each having a hemispherical cavity, for 20 minutes to prepare sphericalcores having a diameter of 39.8 mm. The heat pressing was performed attemperatures shown in Tables 1 to 3.

TABLE 1 Golf ball No. 1 2 3 4 5 6 7 Core Rubber composition BR730 100100 100 100 100 100 100 (part by mass) Sanceler SR 29 29 29 31 23 27 272-Thionaphthol 0.2 0.2 0.2 — — 0.1 0.2 PBDS — — — 0.63 — — — 2,6-DCTP —— — — — — — Zinc octanoate 7.5 — — — — — — Benzoic acid — 5 — 5 — — —Zinc dibenzoate — — — — — — — 4-Dimethylaminobenzoic acid — — 5 — — — —Zinc oxide 5 5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1) *1) *1)PERCUMYL D — — — — 0.8 0.8 — PERHEXYNE 25B 0.49 0.49 0.49 0.49 — — 0.49PERHEXA 25B — — — — — — — PERBUTYL P — — — — — — — PERHEXA C-40 — — — —— — — PERHEXA HC — — — — — — — Press temperature (° C.) 170 170 170 170170 170 170 Core hardness Center hardness 45.1 46.2 47.1 45.2 56.8 56.254.5 distribution (JIS-C) 12.5% point hardness 50.3 50.2 50.1 48.6 60.762.7 60.4 25% point hardness 54.3 54.0 53.1 52.1 64.5 67.1 63.3 37.5%point hardness 56.5 56.3 56.3 54.4 66.5 68.3 64.2 50% point hardness62.0 61.3 62.2 57.3 67.2 68.5 65.6 62.5% point hardness 69.3 70.6 69.566.2 67.6 68.2 70.2 75% point hardness 70.6 77.7 74.1 78.8 71.3 71.673.8 87.5% point hardness 69.5 77.1 73.0 80.5 72.1 75.4 73.0 Surfacehardness 70.0 75.7 73.3 80.1 80.6 83.9 73.5 Surface hardness − centerhardness 24.9 29.5 26.2 34.9 23.8 27.7 19.0 (H_(37.5) − Ho)/(H_(75.0) −Ho) 0.45 0.32 0.34 0.27 0.67 0.79 0.50 Compression deformation amount(D) (mm) 4.20 4.04 4.15 4.21 4.09 4.06 4.17 −9 × D + 95 57.2 58.6 57.757.1 58.2 58.5 57.4 Cover Type A A A A A A A Slab hardness (Shore D) 6565 65 65 65 65 65 Thickness(mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Golf BallDriver spin rate (rpm) −100 −120 −90 −120 0 0 0 Driver flight distance(m) 2.8 3.5 3.0 3.8 0 0.7 0 Compression deformation amount (mm) 3.503.34 3.45 3.51 3.39 3.23 3.47 *1) As to an amount of barium sulfate,adjustment was made such that the golf ball had a mass of 45.4 g.

TABLE 2 Golf ball No. 8 9 10 11 12 13 14 15 Core Rubber compositionBR730 100 100 100 100 100 100 100 100 (part by mass) Sanceler SR 33 3529 36 37 40 36 34 2-Thionaphthol 0.32 0.2 0.1 0.1 0.1 — — 0.2 PBDS — — —— — — 0.63 — 2,6-DCTP — — — — — 0.22 — — Zinc octanoate 5 — 5 7.5 — — —— Benzoic acid — 5 — — 5.2 5.2 — — Zinc dibenzoate — — — — — — 6.5 —4-Dimethylaminobenzoic acid — — — — — — — — 4-Chlorobenzoic acid — — — —— — — 6.7 Zinc oxide 5 5 5 5 5 5 5 15 Barium sulfate *1) *1) *1) *1) *1)*1) *1) *1) PERCUMYL D 0.8 0.8 — — — — — — PERHEXYNE 25B — — 0.49 0.490.49 0.49 0.49 0.49 PERHEXA 25B — — — — — — — — PERBUTYL P — — — — — — —— PERHEXA HC — — — — — — — — Press temperature (° C.) 170 170 170 170170 170 170 170 Core hardness distribution Center hardness 51.6 43.650.8 51.1 50.4 51.2 47.4 52.0 (JIS-C) 12.5% point hardness 58.2 48.756.1 56.3 54.2 55.2 52.3 55.2 25% point hardness 61.9 55.0 61.1 60.358.3 59.7 56.7 58.6 37.5% point hardness 64.5 58.8 64.0 62.5 59.9 61.759.5 61.6 50% point hardness 66.5 61.3 69.4 68.0 62.2 62.4 60.6 69.362.5% point hardness 72.5 61.4 75.7 75.3 71.5 67.4 70.8 76.9 75% pointhardness 79.1 72.6 78.5 80.2 81.7 80.3 82.7 80.7 87.5% point hardness82.5 80.2 75.9 79.3 82.5 87.5 82.6 79.3 Surface hardness 85.6 87.1 80.183.5 86.7 89.4 83.6 81.4 Surface hardness − 34.0 43.5 29.3 32.4 36.338.2 36.2 29.4 center hardness (H_(37.5) − Ho)/(H_(75.0) − Ho) 0.47 0.520.48 0.39 0.30 0.36 0.34 0.34 Compression deformation 4.08 4.30 3.343.10 3.15 3.27 3.16 3.44 amount (D) (mm) −9 × D + 95 58.3 56.3 64.9 67.166.7 65.6 66.6 64.0 Cover Type A A B B B B B B Slab hardness (Shore D)65 65 47 47 47 47 47 47 Thickness(mm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Golf Ball Driver spin rate (rpm) −40 −30 −90 −100 −120 −120 −110 −60Driver flight distance (m) 1.2 1.0 3.5 3.1 3.5 3.8 3.7 2.5 Compressiondeformation amount 3.38 3.60 3.14 2.90 2.95 3.07 2.96 3.24 (mm) *1) Asto an amount of barium sulfate, adjustment was made such that the golfball had a mass of 45.4 g.

TABLE 3 Golf ball No. 16 17 18 19 20 21 22 Core Rubber composition BR730100 100 100 100 100 100 100 (part by mass) Sanceler SR 34 28 29 31 31 2928 2-Thionaphthol 0.1 0.1 0.1 0.1 0.1 — 0.1 PBDS — — — — — — — 2,6-DCTP— — — — — — — Zinc octanoate — — — — — — — Benzoic acid 5.2 5.2 5.2 5.25.2 — 5.2 Zinc dibenzoate — — — — — — — 4-Dimethylaminobenzoic acid — —— — — — — Zinc oxide 5 5 5 5 5 5 5 Barium sulfate *1) *1) *1) *1) *1)*1) *1) PERCUMYL D 0.8 — — — — 0.8 0.8 PERHEXYNE 25B — — — — — — —PERHEXA 25B — 0.5 — — — — — PERBUTYL P — — 0.63 — — — — PERHEXA C-40 — —— 0.87 — — — PERHEXA HC — — — — 0.6 — — Press temperature (° C.) 150 150150 130 130 170 170 Core hardness Center hardness 45.6 47.2 48.2 48.147.8 65.9 62.1 distribution (JIS-C) 12.5% point hardness 51.7 54.1 53.854.0 55.6 70.0 64.4 25% point hardness 57.1 57.8 57.5 59.6 61.9 73.168.0 37.5% point hardness 59.5 60.5 59.6 63.8 64.2 74.1 72.1 50% pointhardness 61.1 65.7 63.3 68.8 66.5 74.3 74.3 62.5% point hardness 66.677.8 74.0 74.3 72.4 73.6 74.6 75% point hardness 80.8 81.2 82.2 77.778.0 75.6 74.8 87.5% point hardness 84.3 80.2 82.7 77.5 79.5 78.8 74.1Surface hardness 90.8 84.9 86.5 83.1 84.5 85.0 73.1 Surface hardness −center hardness 45.2 37.7 38.3 35.0 36.7 19.1 11.1 (H_(37.5) −Ho)/(H_(75.0) − Ho) 0.39 0.39 0.34 0.53 0.54 0.85 0.79 Compressiondeformation amount (D) (mm) 3.08 3.33 3.24 2.95 3.29 3.29 3.21 −9 × D +95 67.3 65.0 65.9 68.5 65.4 65.4 66.1 Cover Type B B B B B B B Slabhardness (Shore D) 47 47 47 47 47 47 47 Thickness(mm) 1.5 1.5 1.5 1.51.5 1.5 1.5 Golf Ball Driver spin rate (rpm) −100 −80 −70 −50 −60 0 −30Driver flight distance (m) 3.1 2.9 2.6 2.4 2.1 0 1.2 Compressiondeformation amount (mm) 2.88 3.13 3.04 2.75 3.09 3.09 3.01 *1) As to anamount of barium sulfate, adjustment was made such that the golf ballhad a mass of 45.4 g. BR730: a high-cispolybutadiene (cis-1,4 bondcontent = 96 mass %, 1,2-vinyl bond content = 1.3 mass %, Moonyviscosity (ML₁₊₄ (100° C.) = 55, molecular weight distribution (Mw/Mn) =3) available from JSR Corporation Sanceler SR: zinc acrylate (product of10 mass % stearic acid coating) available from Sanshin Chemical IndustryCo., Ltd. 2-Thionaphthol: available from Tokyo Chemical Industry Co.,Ltd. PBDS: bis(pentabromophenyl) disulfide 2,6-DCTP:2,6-dichlorothiophenol Zinc octanoate: available from Mitsuwa ChemicalsCo., Ltd. (purity 99 mass % or more) Benzoic acid: available fromSigma-Aldrich Co., Ltd. (purity 99.5 mass % or more) Zinc dibenzoate:available from Wako Pure Chemical Industries, Ltd. (purity 95 mass % ormore) 4-Dimethylaminobenzoic acid: available from Tokyo ChemicalIndustry Co., Ltd. (purity 98 mass % or more) 4-Chlorobenzoic acid:available from Tokyo Chemical Industry Co., Ltd. (purity 99 mass % ormore) Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd. Bariumsulfate: “Barium sulfate BD” manufactured by Sakai Chemical IndustryCo., Ltd., adjustment was made such that the finally obtained golf ballhad a mass of 45.4 g. Dicumyl peroxide: “PERCUMYL ® D” (one-minutehalf-life temperature = 175.2° C.) available from NOF Corporation.2,5-Dimethyl-2,5-bis(t-butylperoxy)hexyne-3: “PERHEXYNE ® 25B”(one-minute half-life temperature = 194.3° C.) available from NOFCorporation. 2,5-Dimethyl-2,5-bis(t-butylperoxy)hexane: “PERHEXA ® 25B”(one-minute half-life temperature = 179.8° C.) available from NOFCorporation. Di(2-t-butylperoxyisopropyl)benzene: “PERBUTYL ® P”(one-minute half-life temperature = 175.4° C.) available from NOFCorporation. 1,1-Di(t-butylperoxy)cyclohexane: “PERHEXA C-40”(one-minute half-life temperature = 153.8° C.) available from NOFCorporation. 1,1-Di(t-hexylperoxy)cyclohexane: “PERHEXA HC” (one-minutehalf-life temperature = 149.2° C.) available from NOF Corporation.

(2) Production of Cover

Cover materials shown in Table 4 were extruded with a twin-screwkneading extruder to prepare the cover compositions in the pellet form.The extruding conditions of the cover compositions were a screw diameterof 45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and themixtures were heated to 150 to 230° C. at the die position of theextruder. The obtained cover compositions were injection-molded onto thespherical core obtained above to produce the golf ball comprising thespherical core and the cover covering the spherical core.

TABLE 4 Cover composition A B Formulation Himilan 1605 50 — Himilan 170650 — Elastollan XNY97A — 100 Titanium oxide 4 4 Slab hardness (Shore D)65 47

Formulation: Parts by Mass

Himilan 1605: Sodium ion neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., LtdHimilan 1706: Zinc ion neutralized ethylene-methacrylic acid copolymerionomer resin available from Du Pont-Mitsui Polychemicals Co., LtdElastollan XNY97A: Thermoplastic polyurethane elastomer available fromBASF Japan Ltd.

Evaluation results of the golf balls are shown in Tables 1 to 3. Thespherical cores of the golf balls No. 1 to No. 4 and No. 10 to No. 20have a hardness distribution satisfying the requirements (I) to (III).All of these golf balls have a lowered spin rate on driver shots and anenhanced flight distance.

The golf ball of the present invention has a lowered spin rate on thedriver shots and an excellent flight performance. This application isbased on Japanese Patent applications No. 2013-115473 filed on May 31,2013, and No. 2014-039187 filed on Feb. 28, 2014, the content of whichare hereby incorporated by reference.

1. A golf ball comprising a spherical core and at least one cover layercovering the spherical core, wherein when JIS-C hardness of thespherical core is measured at points located at distances of 0% (corecenter), 37.5%, 75.0% and 100% (core surface) from the core center ofthe spherical core, a hardness difference (Hs−Ho) between a surfacehardness (Hs) thereof and a center hardness (Ho) thereof is 20 or more,a ratio ((H_(37.5)−Ho)/(H_(75.0)−Ho)) of a hardness difference(H_(37.5)−Ho) between a 37.5% point hardness (H_(37.5)) and a centerhardness (Ho) to a hardness difference (H_(75.0)−Ho) between a 75.0%point hardness (H_(75.0)) and a center hardness (Ho) is 0.6 or less, anda compression deformation amount (D (mm)) of the spherical core whenapplying a load from an initial load of 98N to a final load of 1275 N tothe spherical core and the 37.5% point hardness (H_(37.5)) satisfy arelation of H_(37.5)≦−9×D+95.
 2. The golf ball according to claim 1,wherein the hardness difference (Hs−Ho) is 25 or more in JIS-C hardness.3. The golf ball according to claim 1, wherein the spherical core isformed from a rubber composition containing (a) a base rubber, (b) anα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or ametal salt thereof as a co-crosslinking agent, (c) a crosslinkinginitiator and (d) a carboxylic acid and/or a salt thereof.
 4. The golfball according to claim 3, wherein the rubber composition contains (d)the carboxylic acid and/or the salt thereof in an amount ranging from0.5 part to 30 parts by mass with respect to 100 parts by mass of (a)the base rubber.
 5. The golf ball according to claim 3, wherein (c) thecrosslinking initiator is a peroxy ketal and/or a dialkyl peroxide. 6.The golf ball according to claim 3, wherein the rubber compositionfurther contains (f) an organic sulfur compound.
 7. The golf ballaccording to claim 6, wherein (f) the organic sulfur compound includesat least one compound selected from the group consisting of thiophenolsand/or metal salts thereof, thionaphthols and/or metal salts thereof,diphenylpolysulfides and thiuramdisulfides.
 8. The golf ball accordingto claim 6, wherein a content of (f) the organic sulfur compound is 0.05part to 5 parts by mass with respect to 100 parts by mass of (a) thebase rubber.
 9. The golf ball according to claim 3, wherein a content of(b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atomsand/or the metal salt thereof is 15 parts to 50 parts by mass withrespect to 100 parts by mass of (a) the base rubber.
 10. The golf ballaccording to claim 3, wherein the rubber composition contains (b) themetal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms as (b) the co-crosslinking agent.
 11. The golf ball according toclaim 1, wherein the ratio ((H_(37.5)−Ho)/(H_(75.0)−Ho)) is 0.55 or lessin JIS-C hardness.
 12. The golf ball according to claim 3, Wherein acontent of (c) the crosslinking initiator is 0.2 part to 5 parts by masswith respect to 100 parts by mass of (a) the base rubber.